1. Field of the Invention
The present invention relates to photo-alignment materials useful in liquid crystal alignment films. More particularly, the present invention relates to photo-alignment materials useful in liquid crystal alignment films that include a uniform alignment of liquid crystals. The materials of the invention can be used in a liquid crystal display device to improve electrical and electrooptical properties of the alignment film and thus improve the reliability of products using the alignment film.
2. Description of the Related Art
The arrangement of liquid crystals in a liquid crystal display device changes in accordance with an electric field induced by an externally applied voltage. Such changes in the alignment of the liquid crystals determine whether external light entering the liquid crystal device is blocked or transmitted. Thus, the liquid crystal device can be driven by this property of the liquid crystals. The quality of a liquid crystal display device as a display device is determined by properties that are varied according to the alignment state of the liquid crystals, including light transmittance, response time, viewing angle, contrast ratio, and the like. Therefore, it is very important to uniformly control the liquid crystal alignment in liquid crystal devices.
An alignment film typically refers to a layer of polymer material, which is formed between liquid crystals, and a transparent conductive film made of indium tin oxide in order to produce the uniform alignment, i.e. orientation of liquid crystalline molecules. After formation, the polymer layer typically is subjected to a mechanical process, such as rubbing and the like, and other processes to control the alignment of liquid crystals.
The method currently used to achieve uniform alignment of liquid crystals or to orient liquid crystals in a given direction in preparing liquid crystal display devices involves disposing a layer of polymer, such as polyimide, on a transparent conductive glass substrate, and rubbing the polymer surface at a high speed with a rotating roller that is wrapped with a rubbing cloth made of nylon or rayon. By this rubbing process, the liquid crystalline molecules are oriented with a specific pretilt angle on the surface of the alignment film.
Since this rubbing process is substantially the only method to orient liquid crystals easily and stably, most manufacturers producing liquid crystal display devices generally use the rubbing process for mass-production. However, the rubbing process has problems in that it creates scratches on the surface of the liquid crystal alignment film due to mechanical rubbing, and it generates static electricity which leads to destruction of thin film transistors. In addition, micro fibers released from the rubbing cloth may cause defects in the liquid crystal devices. Accordingly, this rubbing process reduces the production quality of the devices. A new alignment technique therefore has been proposed that aligns liquid crystals by irradiation of light, for example, UV rays, in order to overcome the problems involved in the rubbing process as described above and thereby to improve productivity.
Recently, liquid crystal displays have become large-scale, and the applications of the liquid crystal display are expanding beyond personal applications, such as notebook computers, to household applications such as wall-mounted TVs. In accordance with this trend, a high quality picture and a wide viewing angle are required for the liquid crystal display devices. Also, in order to meet such demands for qualities of the liquid crystal display, the photo-alignment method is currently in the spotlight.
However, the photo-alignment methods reported by M. Schadt et al. (Jpn. J. Appl. Phys., Vol. 31, 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys., Vol. 34, 1992, L1000) are not yet commercialized, in spite of the superiority of its concept, because there is difficulty in developing novel materials to support these methods. One of the major reasons for the difficulty is that raw materials of the alignment film are not sufficiently processible to be applied to the conventional method for manufacturing liquid crystal display devices. Also, the display device using the alignment film formed by photo-alignment is inferior in display quality, as compared to the display device formed using an alignment film of polyimide by rubbing process.
The present inventors have suggested a photo-alignment materials having side chains bearing cinnamate groups attached to the base homopolymer or copolymer chain of maleimide. However, in practice, there is still required an improvement in electrical properties and electrooptical properties of the alignment materials to be applicable to liquid crystal display devices.
The disadvantages and deleterious properties described above with reference to certain materials, devices, methods, and apparatus is not intended to limit the present invention. Indeed, certain features of the invention may include any or all of the materials, devices, methods, and apparatus, without suffering from the disadvantages and deleterious properties so described.